Solid oxide fuel cell structures

ABSTRACT

Solid oxide fuel cell structures which are capable of relatively rapid temperature changes without cracking and which are simple to seal. In one arrangement, a tubular SOFC structure comprises a self-supporting extruded tube of zirconium oxide with inner and outer electrodes. The tube may have an outside diameter of from, for example, 1 to 5 mm and a wall thickness of from, for example, 50 to 200 microns. In an alternative arrangement, a simple gas type planar interconnect for a planar SOFC is provided in the form of a sheet of ceramic material having electrically conducting bodies of ceramic material embedded in it so as to provide an electrical path through the sheet. To avoid edge sealing problems, fuel gas and air may be delivered to a stacked SOFC structure through tubes extending between adjacent cell sub-assemblies such that the gas is delivered to a central portion of each anode of the cell stack and flows outwards towards the edges of the stack.

This is a National Stage Application of PCT/GB94/00549 filed Mar. 17,1994.

The present invention relates to solid oxide fuel cell (hereinafterreferred to as SOFC) structures.

BACKGROUND OF THE INVENTION

SOFCs of three basic designs are currently being developed. These threebasic designs are generally referred to by the term tubular, planar andmonolithic. All of these fuel cells are based upon a stabilised zirconiaelectrolyte which is capable of conducting oxygen ions at elevatedtemperatures. A typical operating temperature for a SOFC is 1000° C. Theknown cells provide high electrical efficiencies and can be operated ona variety of fuels including hydrogen, carbon monoxide, coal-derivedgases and natural gas. SOFCs also offer high quality exhaust heat forco-generation applications. Of potentially the greatest significance,however, is the fact that SOFCs produce very low emissions as comparedwith, for example, diesel generators and therefore can be locatedwherever an electrical generator is required. For example, it would bepossible to replace a relatively dirty and noisy diesel generatorproviding power to a hospital by a SOFC.

The paper "Solid Oxide Fuel Cells--The Next Stage", author Brian Riley,pages 223-238 of "Journal of Power Sources", 29(1990) briefly describesthe various known SOFC structures and reference should be made to thatdocument for details of the structure of the known planar, monolithicand tubular geometries. It will be appreciated, however, that all of theknown structures incorporate a solid electrolyte one side of whichsupports an anode to which fuel gas is delivered and the other side ofwhich supports a cathode to which air or oxygen is delivered. When anexternal load is connected to the anode and cathode, oxygen at thecathode reacts with incoming electrons from the external circuit to formoxygen ions which migrate to the anode through the oxygen-ion conductingelectrolyte. At the anode, the fuel is oxidised with these oxygen ions,resulting in the liberation of electrons to the external circuit. Thusthe overall reaction is simply the oxidisation of fuel. Typically 50 to90% of the fuel is utilised in the electrochemical cell reaction, thepartially depleated fuel being combusted outside the cell. The exhaustgas from the cell can be used in a co-generation system for producingprocess steam or in a steam turbine for an all-electric system. As eachcell has a theoretical open voltage of about 1 volt, it is necessary tointerconnect a number of cells to provide an appropriate output voltage.

If SOFCs are to be widely useable, they must be very reliable over longterm use. The known cells are prone to two problems which compromiselong term reliability, the first problem being the fact that fuel cellstructures are very easily damaged if subjected to thermal shocks, andthe second problem being related to the difficulty experienced insealing the fuel cell structures so that fuel and oxygen are reliablydelivered to opposite sides of a relatively thin electrolyte and do notcome into contact with each other until the fuel has been at leastsignificantly depleated. It has proved difficult to deal with theseproblems given the high operating temperatures and the fact that it isfundamental to the operation of fuel cells that thin ceramic structuresform the interface between the anode and cathode. Such structures crackeasily when exposed to varying temperatures. The conventional approachto reducing the significance of these problems is to very slowly heat upthe fuel cell structure to the normal operating temperature of 1000° C.and to maintain that temperature continuously. Unfortunately in the realworld continuous operation of a system cannot be guaranteed. Until suchtime as manufacturers can assure potential users that, for example, apower failure resulting in rapid cooling of a SOFC would not cause anystructural damage to such a cell it is going to be very difficult toconvince potential customers that SOFCs are a viable alternative toconventional electrical generation systems.

In the case of conventional tubular electrode structures, the basic cellis in the form of a porous support tube onto which a cathode or airelectrode is deposited as a layer by slurry-dipping. The electrolyte isthen deposited on the cathode by electrochemical vapour deposition orplasma spraying. The anode or fuel electrode is then formed on theelectrolyte by slurry dipping. Electrical connections are made to theanode by a simple nickel felt pad applied to its outer surface.Electrical connections are made to the cathode by forming an elongatestrip of electrical conductor along the length of the tube, theelectrolyte not covering the electrical conductor. Tubes can thus beinterconnected by appropriately positioning them with the cathode andanode interconnects of adjacent tubes in contact. The exterior surfaceof the tube is exposed to fuel gas, and air is pumped into the interiorthrough an air tube extending along most of the length of the tube fromone end of the tube. The other end of the tube is closed so thatinjected air flows back through the annular space defined between theair tube and the support tube.

The known tubular arrangement is effective but cannot be heated orcooled rapidly without cracking. The incorporation of the axiallyextending air tube simplifies the problem of sealing, as a seal can bemade relatively easily to the relatively cool air tube, and in additionenables some preheating of the air delivered to the interior of thesupport tube, thereby reducing thermal shocks. Unfortunately, providingthe air tube increases costs substantially both because of increasedmaterial costs and because the structure is relatively difficult tomanufacture. Furthermore, the deposition of the electrolyte andinterconnect on the cathode is a very costly process step.

The manufacturers of the known tubular structures have recognised thatthe incorporation of the support tube accounts for some 70% of the totalweight of the cell which results in a relatively low energy density forthe design. With a view to reducing the overall weight, it has beenproposed to replace the calcia-stabilised support tube by aself-supporting cathode to improve the energy density. This approach mayimprove the energy density but does not address the problems of crackingor cost outlined above.

In the case of planar and monolithic SOFCs, which comprise a stack ofplate-like sub-assemblies, it is conventional practice to feed air intothe structure from a manifold located on one side of the stack and tofeed fuel gas into the structure from a manifold located on an adjacentside of the stack. The fuel and air are pumped in mutually perpendiculardirections from one side of the stack towards the other. Seals must beprovided around the edges of the manifolds, around the edges of theanodes which face and must be isolated from the air manifold, and aroundthe edges of the cathodes which face and must be isolated from the fuelmanifold. The formation of these seals is difficult to achieve reliably,and the seals are prone to cracking. for example, in the event of thefuel cell structure being allowed to cool.

In the case of planar SOFC stacks, a further problem is encountered inproviding the interconnection between adjacent cells. One approach whichhas been used is to form a metal plate into a corrugated sheet. Suchplate tends to oxidise, however, and thus form insulating corrodedlayers, or they expand in a different manner from the adjacent cellplates causing stresses and possible cell fracture. As an alternative tothe metal interconnects, proposals have been made to fabricateinterconnects from lanthanum chromite, but these have provide too bulkyand difficult to fabricate economically. Attempts have been made to usevapour deposited lanthanum chromite as the interconnect but again thishas proved very expensive to manufacture.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate theproblems outlined above.

According to the present invention there is provided a tubular SOFCstructure, comprising a self-supporting extruded tube at least alongitudinally extending portion of which is formed from an electrolyte,an electrically conductive inner electrode making electrical contactwith the inner wall of the electrolyte, and an electrically conductiveouter electrode making electrical contact with the outer wall of theelectrolyte.

By relying upon the electrolyte structure itself to be self-supportingthe tube may be made with a sufficiently small diameter and small wallthickness to make it highly resistant to cracking. For example a tube ofthis structure can be heated at any point along its length with a verylocalised flame to the normal operating temperatures of an SOFC(typically 1000° C.) without incurring any damage. Thus an SOFC based onsuch tubular components can be rapidly heated and cooled without damage,dramatically improving the reliability of such systems and making themuseable in many more applications, for example as emergency powersupplies.

The tube may be made from stabilised zirconia and readily extruded afterbeing mixed with, for example, polyvinyl butyral and cyclohexanone. Themanufacturing process is thus essentially very simple. Thus the cost ofthe tubes is very much lower than with the known tubular structures.

The inner electrode may extend the length of the extruded tube, althougheach tube could be in effect split into a series of separate cellstructures providing appropriate connections through the tube wall couldbe made. The inner electrode can be in the form of a spiral wire incontact with a porous layer of conductive ink deposited inside the tube.The outer electrode may comprise a porous layer of for example dopedlanthanum manganite engaged by a spiral wire.

Alternatively the inner electrode could be extruded as part of theself-supporting tube and may be formed, for example from a mixture ofnickel and zirconium oxide. The self-supporting tube may be made fromelectrolyte and a longitudinally extending strip of electricallyconductive material that extends radially through the tube wall andmakes contact with the inner electrode. This facilitates interconnectionof a series of the tubes.

The tubular structures may be incorporated in a fuel cell system inwhich a thermally insulating enclosure is provided into which thetubular structures extend and to which fuel gas is supplied. The ends ofthe tubular structures within the enclosure are open to enable residualfuel gas to enter the enclosure. At least one air inlet is providedthrough which air is supplied to the interior of the enclosure, residualair combusting with the residual fuel gas and the resultant combustionproducts passing to an exhaust outlet. The air inlet may pass into theenclosure through the exhaust outlet to preheat incoming air. Air may bemixed with the fuel gas before it is delivered to the tubular structureto prevent the formation of carbon deposits within the reactor tubes.

In accordance with a second aspect of the present invention, there isprovided a gas tight planar interconnect for a planar SOFC, comprising asheet of ceramic material, and a plurality of electrically conductingbodies of ceramic material extending through the sheet to provideconductive paths through the sheet.

The ceramic sheet and the electrically conductive bodies embedded inthat sheet may be readily fabricated and may have very similarcoefficients of expansion such that the interconnect is not readilycracked as a result of changing temperatures. The bodies of electricallyconducting ceramic material may project from both surfaces of the sheetso as to provide a spacing around the projecting portions through whichfuel gas or air may be delivered to an adjacent SOFC. The sheet may be acomposite structure formed on one side from a material which isresistant to air and on the other side from a material which isresistant to fuel gas.

According to a third aspect of the present invention, there is provideda stacked SOFC structure, comprising a series of sub-assemblies eachincluding a plate of electrolyte sandwiched between an anode and acathode, the anode of one sub-assembly being electrically connected tothe cathode of an adjacent sub-assembly in the stack, and passagewaysbeing defined between adjacent sub-assemblies in the stack through whichfuel gas and air are delivered to the anode and cathode, wherein fuelgas supply conduits extend between adjacent sub-assemblies to deliverfuel gas to central regions of each anode such that the delivered gasflows outwards from the central regions of the anodes towards the edgesof the stack.

By arranging for fuel gas to be delivered to the centre of each anode,and for the fuel gas to flow outwards towards the edges of the stacksuch that only residual fuel gases reach the edge of the stack, thesevere problems of edge sealing in stacked SOFCs which have beenexperienced in the past are simply avoided.

Preferably air or oxygen is delivered to central regions of the cathodessuch that the air flows outwards towards the stack edges. Residual fuelgas and oxygen-depleated air reaching the edges of the stack may beburned to maintain the stack temperature. The fuel gas and air may bedelivered to the stack through ceramic tubes of extruded zirconia oxideof the type described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will not be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a section through a tubular SOFC structure in accordance withthe present invention;

FIG. 2 is a section through the structure of FIG. 1 after the structureof FIG. 1 has had internal and external spiral wires attached to it;

FIG. 3 is a schematic illustration of an SOFC system of rudimentary formwhich has been used to prove the utility of the tubular structureillustrated in FIGS. 1 and 2;

FIG. 4 illustrates to a fuel cell system which may be fabricated using aplurality of the tubular structures illustrated in FIGS. 1 and 2;

FIG. 5 illustrates the structure of a further embodiment of theinvention with a co-extruded internal electrode.

FIG. 6 illustrates an alternative arrangement to that of FIG. 5 with aco-extruded electrical interconnect provided in the wall of the tube toenable the interconnection of a series of tubular structures;

FIG. 7 is an exploded view of two gas-tight planar interconnects inaccordance with the present invention and an SOFC planar structure whichin use is sandwiched between the interconnects;

FIG. 8 is a section through the components of FIG. 7 assuming that theyhave been brought together into their normal potential relationship;

FIG. 9 is a schematic illustration of a stacked SOFC structure which canbe built up using components such as those illustrated in FIGS. 7 and 8;and

FIG. 10 illustrates a further stacked SOFC structure in which fuel gasand air are supplied to central regions of the structure to avoidproblems with edge sealing of such stacks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, the illustrated arrangement comprises anextruded electrolyte tube 1 supporting an inner electrode 2 in the formof a nickel/zirconia cermet ink defining an anode. A strontium dopedlanthanum manganite layer 3 is formed on the outside of the tube todefine a cathode or air electrode. As shown in FIG. 2, electricalcontact is made with the internal electrode 2 by winding a spiral wire 4inside the tube. Electrical contact to the outer electrode 3 issimilarly achieved by winding a spiral wire 5 onto the layer 3. If sucha tube is placed in an oxygen-containing enclosure maintained at about1000° C. and fuel gas is supplied to the interior of the tube a solidoxide fuel cell (SOFC) is formed and as a result current will flow toany external circuit connected to wires 4 and 5.

Tubes of the type described have been found to be easy to manufacture,to resist cracking even if heated or cooled rapidly, and to be easy toseal. The tubes can be readily extruded using a mix of, for example,zirconia powder and polymer formulation. After firing to full density,these tubes can be heated to 1000° C. rapidly and cooled equally rapidlywithout damage. Furthermore, the tubes can be passed through aninsulating layer defining the wall of a chamber, the interior of whichis maintained at 1000° C. even though one end of the tube is at roomtemperature and the other end is at the temperature of the enclosure.Cold gas can then be passed down the tube without causing it to crack.Sealing the tube on the cold end is simple, for example using simpleplastics connections. Thus the problems associated with known tubularSOFC structures are overcome.

The thin walled zirconia tube may be made from stabilised zirconia, forexample using yttria stabiliser. Between 3 and 12 mol % may be usedalthough preferably from 6 to 10 mol % is used and the preferred amountof stabiliser is 8 mol %. It will be appreciated that stabilisers otherthan yttria may be used, for example magnesia, calcia, ceria, aluminaand others known in the art. The outside diameter of the tube may befrom 1 to 5 mm, although other sizes are possible. The tube need notnecessarily be round, as fluted or "wavy" cross sections can prove to beadvantageous and are readily extrudable. The zirconia wall thickness ispreferably between 50 and 200 microns (10⁻⁶ meters) to allow for theready passage of oxygen ions during fuel cell operation. If the zirconiais extruded with the anode, the zirconia could be relatively thin, forexample from 5 to 10 microns. The inner and outer electrodes may bedeposited in the form of inks containing active powders. The inks aresintered and then electrical connections are made to them to enablecurrent to be drawn from the cell. Metal or ceramic components may beused to make the electrical connections, for example the illustratedwires shown in FIG. 2. Each tube may form a single cell, althoughseveral cells could be positioned on each tube. In such a case,interconnects would have to be positioned along the length of the tube.Alternatively, a line of interconnect can be formed in the wall of thetube and this can be achieved by extrusion in a single shot process.

Many tubes must be interconnected to make an SOFC assembly of reasonableoutput power. The necessary interconnections may be achieved in a numberof ways. For example, each tube may be a single cell which is connectedexternally to neighboring cells, or each tube may form multiple cellswith internal interconnections along the length of the tube, each tubethen being connected externally to other tubes. As a further alternativeeach tube may be single cell which is connected internally tosurrounding tubes by means of interconnect strips extending along thelengths of the tubes. Tubes may be connected in series or parallel or incombinations of series and parallel connections. The tubes will normallybe straight and parallel, but could also be curved or shaped so as tochannel gas in desirable directions. Fuel gas may be delivered either tothe inside or the outside of the tubes and residual fuel gas may berecycled to the fuel gas inlet.

A tube of the type illustrated in FIGS. 1 and 2 was fabricated using 8mol % yttria stabiliser zirconia powder which was mixed with polyvinylbutyral and cyclohexanone in proportions by weight of 100/8/9. Thecomposition was mixed intensively in a Dow mixer to break down anyagglomerates. The plastics mix was pressed into a sheet under 5 MPapressure and then extruded in a tube die to give a 5 mm diameter tube of0.2 mm wall thickness. After drying, the tube was supported inside analumina tube of slightly larger diameter and fired in a furnace. Thepolymer binder was first removed at 1° C. per minute until a temperatureof 500° C. was achieved, and then the tube was sintered to 1500° C. at5° C. per minute and held at that temperature for one hour.Nickel/zirconia cermet ink was coated on the inside of the tube to formthe anode and strontium doped lanthanum manganite was coated on theoutside to form the air electrode. These electrodes were fired at 1200°C. with platinum wires attached as current leads.

The resultant tube was then placed in a furnace of the typeschematically illustrated in FIG. 3. The tube 6 was passed through abore drilled in a thermally insulating plug 7 inserted in the top of atubular body 8 of insulating material. An exhaust outlet tube 9 was alsopositioned in the plug 7. The tubular body 8 was supported on a base 10.An air inlet tube 11 extended into the lower portion of the enclosuredefined within the insulating body.

A rubber tube 12 was connected to the upper end of the SOFC tubularstructure 6. Fuel gas was then passed through the tube 12 and the tube 6into the enclosure and burnt with air delivered through tube 11. Whenthe temperature within the enclosure had reached 1000° C. the tube wasseen to act as an SOFC. Turning off the supply of fuel gas resulted in arapid fall in temperature within the enclosure but this did not damagethe tube 6, nor did the tube suffer any damage as a result of the largetemperature differentials between its two ends.

FIG. 4 illustrates a proposed system incorporating tubes of the typeillustrated in FIGS. 1 to 3 in an assembly capable of providingreasonable power output levels. An array of tubes 13 is supported withina thermally insulating container 14 from which combustion products canescape through exhaust passageway 15. Fuel gas is supplied to thecontainer 14 through a valve 16, a manifold 17 and the tubes 13, thebottom ends of which simply open into the interior of the container. Airis supplied to the container from a blower fan 18 via an inlet pipe 19that extends through the exhaust outlet 15. The air is pre-heated as itpasses through the tube 19. A water heat exchanger 20 is also providedin the exhaust outlet 15 to extract heat energy from the exhaust gasstream.

The internal electrodes (not visible in FIG. 4) within the tubes 13 areconnected to a negative terminal 21 of a battery 22 whereas the outerelectrodes of the tubes 13 are connected to the positive terminal 23 ofthe battery 22. The output of the battery is converted from DC to AC bya converter 24 to deliver the required AC output as represented by arrow25. The system is controlled by a controller 26 that monitors thecondition of the battery, the temperature within the container 14 asindicated by a thermocouple connected to terminal 27, and the operationof the DC to AC converter 24. In addition the controller 26 energisesthe valve 16 and an igniter positioned within the container 14 andconnected to terminal 28.

The illustrated system was designed to provide both electric power andheat in the 0.2 to 20 KWh scale. The system is well integrated to givethe smallest number of parts, and those parts are designed to enable thesystem to be switched on and off as required without damage. Such asystem is ideally suited to replace conventional power generators suchas diesel or turbine engines.

The tubes 13 are fed with fuel from the manifold 17. The rate of supplyof gas is controlled by the vale 16 in response to control signalsgenerated by the controller 26. The controller also controls the blower18, the speed of which is modulated in response to variations in thetemperature within the container 14 as monitored by the thermocouple. Asthe air and fuel are brought together inside the container, the gas isignited by the igniter. Gas flows could be typically in the range of 10to 1000 ml/s, while air flows are typically in the range of 100 to10,000 ml/s.

The output of the blower 18 is connected by an air bleed line 29 to thegas supply line. Mixing air with the supplied fuel gas prevents theformation of carbon within the tubes 13. This is important as given thatthe tubes have small internal diameters they could easily be blocked bycarbon deposits. The rate of supply of air through the bleed line 29will typically be the same as the rate of supply of fuel gas.

The detailed geometry of the assembly of tubes 13 may vary depending onthe size and purpose of the device. For example, a small device maycontain 20 sub-units each containing 30 short tubes 13. A larger devicemay contain 40 sub-units each containing 100 long tubes. The overalllayout of the system will, however, remain the same in both cases.

it will be appreciated that in larger installations several air feedtubes may be appropriate, and that the water heater may be omitted incertain applications, for example where there is a need for hot gas todrive a heater or chiller.

In the arrangement of FIG. 4, partially depleated fuel gas is simplyburnt within the container 14, the combustion products leaving thecontainer through the exhaust 15. It would be possible, however, tore-cycle partially depleated fuel, for example simply by passing thetubes 13 through the bottom wall of the container 14 and recycling thegas within them to the manifold 17. This approach would enable steamreforming to take place, again with a view to preventing carbon build upin the system. Of course, no problems will arise with carbon if the fuelgas is, for example, hydrogen or methanol.

As briefly mentioned above, alternatives are available to the tubularstructure illustrated in FIGS. 1 and 2. In particular, advantages willarise from reducing the number of process steps and the number ofcomponents required to make any particular tube. As illustrated in FIG.5, the process steps necessary to form the inner electrode shown inFIGS. 1 and 2 can be avoided by the simple expedient of co-extruding thezirconia electrode 30 with an anode 31. The anode could be formed fromzirconia and nickel and would enable current to be carried along thelength of the tube. Such an arrangement would avoid the need fordepositing an ink inside the ceramic tube as shown in FIG. 2 and forproviding a contact wire within the tube.

With the arrangements of FIGS. 1 and 5 current is taken from theinternal electrode to one end of the tube. Alternative arrangements are,however, possible and one such arrangement is illustrated in FIG. 6. Inthis arrangement three components are extruded in a single step, that isa zirconia oxide electrolyte 32, an electrically conducting interconnect33, and an internal electrode 34. An outer nickel-containing electrode35 is then formed on the tube in a position so that it does not contactthe interconnect 33. Adjacent tubes of an identical structure can thenbe interconnected in series as shown in FIG. 6 by sandwiching a nickelfelt pad 36 between adjacent tubes.

Referring now to FIGS. 7 to 9, a gas type planar interconnect for aplanar SOFC will be described. A planar SOFC comprises a series ofsub-assemblies each including a plate of electrolyte 37 (for examplezirconia dioxide) sandwiched between a pair of electrodes 38 and 39which respectively define the anode and cathode of the sub-assembly.Fuel gas is supplied to the anode 38 and air or oxygen is supplied tothe cathode 39. Interconnect plates 40 and 41 are disposed above andbelow the electrolyte and electrode structure to convey currentvertically through the stack. Clearly it is necessary to providepassages between the plates 38 and 40 and between the plates 39 and 41to enable the supply of air and fuel gas. Typically this is achieved inprior art planar SOFC devices by forming bipolar interconnect plateswhich define grooves on each of their surfaces, one set of groovesdelivering air to an electrode beneath the plate and the other set ofgrooves delivering fuel gas to an electrode above the plate. Themanufacture of such bi-polar plates is a relatively expensive process.

The interconnect plates 40 and 41 as shown in FIGS. 7 and 8 can besubstituted for the known bi-polar plates. The interconnect plates 40and 41 comprise an insulating ceramic sheet incorporating electricallyconducting ceramic bodies 42. The result is a thin sheet of insulatingmaterial which is also gas tight, mechanically strong and matched in itsthermal expansion characteristics to the ceramic components of theremaining parts of the SOFC. The basic sheet is preferably formed frompartially stabilised zirconia, or some other strong ceramic material.This support sheet is typically from 0.1 to 0.5 mm in thickness. Theconducting elements 42 are spaced apart in a regular array across thesupport plate, the conducting elements typically being 0.5 to 5 mmacross and spaced apart at from 5 to 20 mm intervals. In the illustratedexample, as shown in FIG. 8, the conducting elements project from bothof the surfaces of the support plate so as to provide a spacing betweenthe support sheet of interconnect 40 and the electrode 38 and betweenthe support sheet of interconnect 41 and electrode 39. This spacingenables gas to flow to the electrodes. The electrically conductingelements thus not only provide the required spacings but also provide aroute for current to flow between adjacent sub-assemblies in the stack.

The interconnect plate must resist both air and fuel, and with this inmind may be made from lanthanum chromite. The conducting elements willideally have the same coefficient of expansion as the support plate. Thesupport plate may itself be a composite structure, formed from an airresistant material on one side and a fuel resistant material on theother.

Although the conducting elements in the illustrated case are arranged toproject from the two surfaces of the support plates so as to provide aspacing between the support plate and the adjacent SOFC structure, insome circumstances an interconnect may be required where there is noneed to provide such a spacing. In those circumstances it would beappropriate for the conducting elements to be of the same thickness asthe support plate.

The interconnect plate need not be flat, but could be convoluted asrequired. The edges of the plate could be shaped to guide gas flows orto provide for sealing.

In one example, a gas tight planar interconnect in accordance with theinvention was formed by mixing 3 mol % yttria stabilised zirconia powderwith polyvinyl butyral and cyclohexanone in proportions by weight of100/8/9. The composition was mixed intensively to break down theagglomerates. The sheet was then pressed and rolled into a sheet 0.3 mmthick, and 1 mm diameter holes spaced 10 mm apart were cut into thesheet with a punch. Pressed spheres of lanthanum chromite powder wereinserted in the holes and sealed into the zirconia sheet by pressure.The sheet was then dried out and the composite was fired at 1550° C. tosinter for one hour. The two materials were matched for sinteringshrinkage, and accordingly the plate did not crack, but performed as aconducting connector between two cells made from cubic zirconia coatedwith nickel cermet and lanthanum manganite electrodes.

Sub-assemblies as exemplified by components 37, 38 and 39 in FIG. 7would in an SOFC stack alternate in the stack with interconnect platessuch as plate 40 or plate 41 of FIG. 7. The final assembly might be asillustrated in FIG. 9, that is a cuboid stack 43 having electricalconductors 44 and 45 connected to its upper and lower levels. An airmanifold 46 would be connected to one side face of the stack, the airmanifold being in communication with one side only of each of theelectrode sub-assemblies. A similar fuel gas manifold 47 would beconnected to an adjacent side face of the stack, the manifold 47 beingin communication with only the cathode sides of the electrodesub-assemblies. The other two sides of the stack would not be sealed soas to enable oxygen-depleated air pumped in through the manifold 46 anddepleated fuel pumped in through the manifold 47 to be removed from thestack. Such an arrangement is a practical possibility but sealing theedges of the stack is difficult to achieve in a reliable manner.Accordingly an alternative arrangement is illustrated in FIG. 10.

FIG. 10 shows three electrode sub-assemblies 48, 49 and 50 separated byplanar interconnects 51 and 52. The sub-assemblies 48, 49 and 50 areidentical to the sub-assembly shown in FIG. 7 and the interconnects 51and 52 are identical to the interconnects 40 and 41 of FIG. 7. In thearrangement of FIG. 10, however, rather than relying upon the supply ofair and fuel gas from edges of the stack. Fuel is injected through fuelinlet tubes 53 and air is injected through air inlets 54. It will beseen that the fuel and air inlets both terminate in central regions ofthe stack such that the injected air and fuel moves from its deliverypoint within the stack towards the edges of the stack. During itsprogress towards the edges of the stack the fuel cell operates to removeoxygen from the air and to depleat the fuel gas. Residual gas reachingthe edge of the stack will simply burn with what is left of the oxygenin the supplied air. There are thus no sealing problems of the typeconfronted in structures of the type shown in FIG. 9. If the gas and airinlet tubes 53 and 54 are extruded ceramic tubes of the general typedescribed above it is a simple matter to connect the cold ends of thesepipes by, for example, rubber tubing to appropriate gas and air suppliesdispite the fact that the delivery ends of the tubes are maintained at atemperature of 1000° C. Accordingly not only is it not necessary toprovide high reliability seals to the edge of the stack, but it is alsoa simple matter to make connects to the gas and air supply tubes.

I claim:
 1. A tubular SOFC structure comprising a self-supportingextruded tube a longitudinally extending portion of which includes anelectrolyte, an electrically conductive inner electrode makingelectrical contact with the inner wall of the electrolyte, anelectrically conductive outer electrode making electrical contact withthe outer wall of the electrolyte, a thermally insulating enclosuredefining a wall through which the tube extends such that a first portionof the tube extends within the enclosure and a second portion of thetube extends outside the enclosure, a first gas supply conduit connectedto the end of the second portion of the tube by a gas-tight seal locatedoutside the enclosure, a second gas supply conduit which opens withinthe enclosure, one of the first and second gases being a fuel gas andthe other containing oxygen, and means for heating the interior of theenclosure to a temperature at which the structure operates as a solidoxide fuel cell, wherein the inner electrode extends the length of theextruded tube, and wherein the inner electrode is a spiral wire incontact with a porous layer of conductive ink deposited inside the tube.2. A tubular SOFC structure according to claim 1, wherein the first gasis fuel gas and the second gas is air, the tube terminates in an openend located within the enclosure, and an exhaust conduit is provided forconveying combustion products from the enclosure.
 3. A tubular SOFCstructure according to claims 1 or 2, wherein the self-supporting tubeis stabilized zirconia.
 4. A tubular SOFC structure according to claim3, wherein the stabilizer is yttria.
 5. A tubular SOFC structureaccording to claim 1, wherein the outer electrode comprises a porouslayer of doped lanthanum manganite.
 6. A tubular SOFC structureaccording to claim 5, wherein the outer electrode comprises a spiralwire in contact with the lanthanum manganite.
 7. A tubular SOFCstructure according to claim 1, wherein the inner electrode is extrudedas part of the self-supporting tube.
 8. A tubular SOFC structureaccording to claim 7, wherein the inner electrode is a mixture of nickeland zirconia oxide.
 9. A tubular SOFC structure according to claim 1,wherein the self supporting tube includes an electrolyte and alongitudinally extending strip of electrically conductive material thatextends radially through the tube wall and makes contact with the innerelectrode.
 10. A tubular SOFC structure according to claim 1, whereinthe extruded tube has an outside diameter of from 1 to 5 mm.
 11. Atubular SOFC structure according to claim 1, comprising a preheaterheated by combustion products in the gas supply conduit through whichthe gas containing oxygen is supplied.
 12. A tubular SOFC structureaccording to claim 1, comprising a water heat exchanger to extract heatfrom combustion products.
 13. A tubular SOFC structure comprising aself-supporting extruded tube a longitudinally extending portion ofwhich includes an electrolyte, an electrically conductive innerelectrode making electrical contact with the inner wall of theelectrolyte, an electrically conductive outer electrode makingelectrical contact with the outer wall of the electrolyte, a thermallyinsulating enclosure defining a wall through which the tube extends suchthat a first portion of the tube extends within the enclosure and asecond portion of the tube extends outside the enclosure, a first gassupply conduit connected to the end of the second portion of the tube bya gas-tight seal located outside the enclosure, a second gas supplyconduit which opens within the enclosure, one of the first and secondgases being a fuel gas and the other containing oxygen, and means forheating the interior of the enclosure to a temperature at which thestructure operates as a solid oxide fuel cell, and further comprisingmeans for mixing air with the fuel gas before it is delivered to thetubular structure.
 14. A tubular SOFC structure according to claim 13,wherein the first gas is fuel gas and the second gas is air, the tubeterminates in an open end located within the enclosure, and an exhaustconduit is provided for conveying combustion products from theenclosure.
 15. A tubular SOFC structure according to claim 13, whereinthe self-supporting tube is stabilized zirconia.
 16. A tubular SOFCstructure according to claim 15, wherein the stabilizer is yttria.
 17. Atubular SOFC structure according to claim 13, wherein the innerelectrode extends the length of the extruded tube.
 18. A tubular SOFCstructure according to claim 17, wherein the inner electrode is a spiralwire in contact with a porous layer of conductive ink deposited insidethe tube.
 19. A tubular SOFC structure according to claim 13, whereinthe outer electrode comprises a porous layer of doped lanthanummanganite.
 20. A tubular SOFC structure according to claim 19, whereinthe outer electrode comprises a spiral wire in contact with thelanthanum manganite.
 21. A tubular SOFC structure according to claim 13,wherein the inner electrode is extruded as part of the self-supportingtube.
 22. A tubular SOFC structure according to claim 21, wherein theinner electrode is a mixture of nickel and zirconia oxide.
 23. A tubularSOFC structure according to claim 13, wherein the self supporting tubeincludes an electrolyte and a longitudinally extending strip ofelectrically conductive material that extends radially through the tubewall and makes contact with the inner electrode.
 24. A tubular SOFCstructure according to claim 13, wherein the extruded tube has anoutside diameter of from 1 to 5 mm.
 25. A tubular SOFC structureaccording to claim 13, comprising a preheater heated by combustionproducts in the gas supply conduit through which the gas containingoxygen is supplied.
 26. A tubular SOFC structure according to claim 13,comprising a water heat exchanger to extract heat from combustionproducts.